1. Field of the Invention
The invention relates to an image sensor. More particularly, the invention concerns a structure for an image sensor integrated with the semiconductor device having enhanced blue light transmittance.
2. Description of the Related Art
Charge coupled devices (CCD) are currently used as image sensors. CCD technology has been developed over many years and is now mature and stable. Complementary metal oxide semiconductor (CMOS) image sensor technology is newer than CCD technology but is lower in resolution and quality than a CCD. However, a CMOS image sensor still has other advantages such as lower fabrication costs due to the use of the CMOS fabrication process.
Although the technology for the CMOS image sensor is not stable and most of processes are still being researched, when compared with the CCD, the CMOS image sensor is more easily integrated with a wafer, for purposes such as image processing. Accordingly, the integration of IC devices for a CMOS image sensor can therefore be greatly increased. Fabrication costs are thereby brought down, dimensions are reduced, and power consumption is decreased. All these advantages enlarge the value of an IC device. Therefore, it is predicted that the CMOS image sensor will take the place of the CCD and play a main role in the future.
However, with respect to a CMOS image sensor, transmittance of light for the semiconductor structure used in a semiconductor image sensor is an important factor that seriously influences the quality of the image sensor. For example, it is imperative that the light transmittance is high enough. Only a high transmittance enables the light to arrive at the depletion region with a sufficiently high electric field in the semiconductor substrate. Upon arrival, the transmitted light induces electron-hole pairs due to excitation of photo-energy and thereby produces current in the intrinsic depletion region when light with varied wavelengths penetrates the passivation layer protecting the semiconductor structure.
Referring to FIG. 1, a bonding pad 102 is formed over a semiconductor substrate 100 having a CMOS sensor (not shown) to connect with the device on the semiconductor 100. A passivation layer 104 consisting of phosphosilicate glass (PSG) and silicon nitride (SiNx) with a thickness of about 5000 angstroms and about 7000 angstroms, respectively, is formed on the bonding pad 102 to protect the underlying devices from being damaged. Due to the formation of devices on the semiconductor substrate 100, and especially to the presence of the bonding pad 102, the surface of passivation 104 is extremely uneven. It is necessary to form a plain film 106 on the passivation 104 to planarize the passivation layer topography. The plain film 106 can be made from polyimide or acrylic resin, for example, to a thickness of about 16000 angstroms. Thereafter, a color filter 108 is formed on the plain film 106 and light reaches the semiconductor substrate 100 through the color filter 108.
Blue light transmittance by the silicon nitride in the passivation layer 104 is about 70% when the light passes through the blue color filter 108, since wavelength of the blue light, about 460 nanometers, is shorter, as shown in FIG. 2. The curve 200 in FIG. 2 represents the transmittance of the silicon nitride when the light passes through the blue color filter to penetrate the silicon nitride. In addition, transmittance of the plain film 106 as the light passes through is about 95%. As a result, the total transmittance of these two films is merely approximately 66.5%. The poor transmittance of blue light by the silicon nitride causes the semiconductor substrate 100 to receive insufficient light for current induction, leading to erroneous information.
In addition, since the plain film itself is made from polymeric material, polymer is easily produced and covers the wafer when etching the plain film 104 and passivation layer 104 to expose the bonding pad. The etching rate of the layers obviously decreases when the polymer covers the layers that need to be etched. Also, the plain film 106 is almost 16000 angstroms thick. The etching process is difficult to carry out for these reasons. Accordingly, the etching process for the wafer requires at least 6 minutes and preventive maintenance (PM) to keep the reaction chamber clean is required after etching 15 wafers. As the etching time is too long, the interval between preventive maintenances is too short and etchant for etching the plain film 106 is also expensive, there is no potential for using this process to fabricate a semiconductor image sensor in line and produce product in quantity.
This invention therefore provides a semiconductor image sensor whose blue light transmittance is enhanced.
The invention also provides a semiconductor image sensor that can reduce the etching time and prolong the interval between the PMs. As a result, the manufacturing cost is lowered and the semiconductor image sensor is suitable for line production in quantity.
To achieve these objects and advantages, and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is directed towards a semiconductor image sensor including a semiconductor with a bonding pad. An oxide layer is disposed on the semiconductor to cover the bonding pad. A spin on glass (SOG) covers the oxide layer. A silicon-oxy-nitride layer (SiOxNy) covers the SOG, and a color filter is disposed on the silicon-oxy-nitride layer.
This invention utilizes a silicon-oxy-nitride layer and a SOG with high transmittance to replace the silicon nitride passivation layer and the plain film in prior art. Not only the transmittance of the semiconductor image sensor is enhanced, but also the topography of the films underlying the color filter is planar.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.